In the field of accelerometers, it is known to form a small compact accelerometer by etching the relevant parts out of silicon. U.S. Pat. No. 4,574,327 illustrates one version of such a transducer in which a proof mass having a textured surface containing many grooves and apertures extending through the proof mass has its surface tailored in order to achieve the desired frequency response by using the squeeze-film damping phenomenon.
Other forms of micro-accelerometers employ cantilever proof masses that introduce an asymmetry that can give an undesirable cross-axis sensitivity. The preceding '327 patent avoids that asymmetric effect by showing a flexible hinge all around the proof mass so that the response is directed preferentially to an axis perpendicular to the plane of the proof mass.
Preferably, the hinges are mounted in the mid-plane of the proof mass to avoid torques that will couple accelerations on different axes, but accurate location in the mid-plane is difficult.
The problem solved by the present invention is that prior art mid-plane proof masses were formed by boron-doping the top surface of a silicon wafer and then growing an epitaxial layer above the doped surface to a height that matched the thickness of the silicon under the doped layer. This was a very slow and expensive process.
More important, this process invariably leads to induced stresses in the proof mass structure formed thereby, leading to high device temperature sensitivities and lack of device-to-device reproducibility in accelerometer span and bias offset. Alternate etching from both sides of an undoped wafer has been employed to define mid-plane hinges. This process is controlled only by the duration of the etch. However, since the thickness of the proof mass structure is typically 10 mils, while hinge thicknesses are typically 0.1 mils, this technique does not lead to good device-to-device reproducibility on a wafer-to-wafer basis.